A four channel HTS SQUID NDE system

We have constructed an NDE system using an array of four HTS SQUID sensors. The sensor array was fabricated on a SrTiO/sub 3/ bi-crystal substrate. The SQUID pickup loops were held perpendicular to the device under test. The lateral dimension of the SQUID was 20 micrometers and the noise was typically less than 30 pT/Hz/sup 1/2/ at 10 Hz. We used modulation FLL circuits for SQUID readout. The effect by modulation and feedback coupling between adjacent channels was investigated by measuring noise spectrum and output change with and without operating adjacent channels. We applied the system to map the magnetic field generated by current distribution and magnetic inclusions in a working circuit as well as a current carrying metal pattern.